Matrix isolation infrared spectra of nitrosyl halide (XNO)
complexes with HX and X2 (X = Cl, Br) are presented. The r elative
frequency shifts of the HX mode are modest (ClNO · H-Cl, !\u27:..:v/v =
= - 0.045; BrNO·H-Br, Av/v = ·-0.026), indicating weak hydrogen
bonds 1-3 kcal/mo!. These shifts are accompanied by significant
shifts to higher frequencies in the XN-0 stretching mode
(CIN-O·HCl, Av/v = +0.016; BrN-O·HBr, Av/v = +0.011). Similar
.;hifts were observed for the XN-0 · X2 complexes (ClN-0 Cl2,
Aviv = +0.009; BrN-0-Br2, Av/v = +0.013). In all four complexes,
the X- NO stretching mode relative shift is opposite in sign and
about 1.6 times that of the NO stretching mode.
These four complexes are considered to be similar in structure
and charge distribution. The XN- 0 frequency shift suggests that
complex formation is accompanied by charge withdrawal from the
NO bond ranging from about .04 to .07 electron charges. The HX
and X2 molecules act as electron acceptors, drawing electrons out
of the antibonding orbital of NO and strengthening the XN-0
bond. The implications of the pattern of vibrational shifts concerning
the structure of the complexes are discussed
